Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in making industrial and building products; especially gypsum wallboard. It is a plentiful and generally inexpensive raw material which, through a process of dehydration and rehydration, can be cast, molded, or otherwise formed into useful shapes. It is also noncombustible and relatively dimensionally stable when exposed to moisture. However, because it is a brittle, crystalline material which has relatively low tensile and flexural strength, its uses are typically limited to non-structural, non-load bearing and non-impact absorbing applications.
Gypsum wallboard; i.e., also known as plasterboard or drywall, consists of a rehydrated gypsum core sandwiched between multi-ply paper cover sheets, and is largely for interior wall and ceiling applications. Because of the brittleness and low nail and screw holding properties of its gypsum core, conventional drywall by itself cannot support heavy appended loads or absorb significant impact. Accordingly, means to improve the tensile, flexural, nail and screw holding strength and impact resistance of gypsum wallboard and building products have long been, and still are, earnestly sought.
Another readily available and affordable material, which is also widely used in building products, is lignocellulosic material particularly in the form of wood and paper fibers. For example, in addition to lumber, particleboard, fiberboard, waferboard, plywood and “hard” board (high density fiberboard) are some of the forms of processed lignocellulosic material products used in the building industry. Such materials have better tensile and flexural strength than gypsum wallboard. However, they are also generally higher in cost, have poor fire resistance and are not able to provide adequate strength in lower density products. Therefore, affordable means to remove these use-limiting properties of building products made from cellulosic material are also desired.
Previous attempts to combine the favorable properties of gypsum and cellulosic fibers, particularly wood fibers, are described in detail in U.S. Pat. Nos. 5,817,262 and 5,320,677, both herein incorporated by reference, and assigned to the United States Gypsum Company. It is an object of the present invention to improve upon the teachings of the '262 and '677 patents, and provide a gypsum/wood fiber board (GWF) product having improved strength, impact resistance, resistance to screw and nail pullout, and dimensional stability.
In general and as taught by the '262 and '677. patents, the process for making a composite GWF material begins with mixing between about 0.5% to about 30%, and preferably between 3 % to 20%, by weight, wood fibers with the respective complement of ground, uncalcined gypsum. The dry mix is combined with enough liquid, preferably water, to form a dilute slurry having about 70%-95% by weight water. The slurry is processed in a pressure vessel, such as an autoclave, at a temperature of approximately 285 to 305 degrees F., which is sufficient to convert the gypsum to acicular calcium sulfate hemihydrate crystals. It is desirable to continuously agitate the slurry with gentle stirring or mixing to break up any fiber clumps and keep all the particles in suspension. After the hemihydrate has formed and has precipitated out of solution as hemihydrate crystals, the pressure on the product slurry is relieved when the slurry is discharged from the autoclave. It is at this point that any other desired additives are added to the slurry. While still hot, the slurry is added to a head box which distributes the slurry onto a porous felting conveyor. While on the conveyor, the slurry is dewatered by the action of vacuum pumps which draw the water through the felting conveyor, causing a filter cake to form on the conveyors surface. As much as 90% of the uncombined water may be removed from the filter cake by vacuum pumps. The temperature of the heated slurry is maintained at a temperature above about 160 F until it has been substantially dewatered and wet pressed into a board. As a consequence of the water removal, the filter cake is cooled to a temperature at which point rehydration may begin. However, it may still be necessary to provide external cooling to bring the temperature low enough to accomplish the rehydration within an acceptable time.
Before extensive rehydration takes place, the filter cake is preferably wet-pressed into a board of desired thickness and/or density. If the board is to be given a special surface texture or a laminated surface finish, it would preferably occur during or following this step of the process. During the wet pressing, which preferably takes place with gradually increasing pressure to preserve the products integrity, two things happen: (1) additional water, for example about 50%-60% of the remaining water, is removed; and (2) as a consequence of the additional water removal, the filter cake is further cooled to a temperature at which rapid rehydration occurs. The calcium sulfate hemihydrate hydrates to gypsum, so that the acicular calcium hemihydrate crystals are converted to gypsum crystals in-situ in and around the wood fibers. After rehydration is complete, the boards can be cut and trimmed, if desired, and then sent through a kiln for drying. Preferably, the drying temperature should be kept low enough to avoid recalcining any gypsum on the surface.
The GWF product has historically relied on gypsum as the sole internal binder. While the use of gypsum as the sole core binder has worked well for the higher density GWF products such as the 55-pcf (pounds per cubic foot) exterior sheathing and 65-pcf underlayment products, it has not been able to provide adequate strength for lower density products. In particular, the gypsum-only GWF product has proved to possess inadequate flexural strength and stiffness for key low-density products such as furniture components and other wood-based applications. Thus, ways to improve the physical properties of the GWF product have been sought.
Beginning in the early 1980's, aqueous dispersions of isocyanate became commercially available. One of these dispersions, based on the use of a particular diisocyanate, MDI, has gained acceptance as a binder for particleboard and oriented strand board (OSB) in place of conventional phenol-formaldehyde resins. In considering the application of MDI to the GWF product, the MDI emulsion system described herein has a number of advantages over gypsum as a binder. MDI is a thermoset resin capable of forming chemical bonds with both hydroxyl groups on the cellulosic fibers as well as cross-linking with itself by reaction of the isocyanate group with water. Through judicious use of catalysts, one can preferentially catalyze either of those reactions if desired. MDI is also resistant to moisture and humidity and, if properly employed in the GWF panel, provides improved physical properties, such as flexural and tensile strength, resistance to nail and screw pull out and impact resistance.
Any binder, such as MDI, added to the GWF slurry must satisfy two conditions: (1) it must be stable in the temperature and chemical conditions present in the headbox, and (2), the binder must be retained in the basemat. These requirements have thwarted previous attempts to incorporate MDI into the GWF product.
Previous unsuccessful attempts have been made to utilize neat MDI as a core additive in a GWF product at a low level, approximately 2% by weight based on the total solids in the slurry. In the previous attempts, the neat MDI immediately reacted with water in the headbox and polymerized into a brown polymeric material resulting in the MDI being retained in the GWF basemat in the form of a localized polyurea solid. This immediate reaction and localization experienced in the previous attempts is undesirable since the MDI is not acting as a binder but is contained within the core of the product as an inert filler. In this condition, the MDI does not render an increase in strength to the GWF product. Tests have also shown that utilizing a premix of MDI and water leads to similar disappointing results because of its very limited stability.
The successful use of MDI in the GWF process rests on the MDI being stable in the head box so that it polymerizes some time later in the board forming process, and it rests on the MDI being retained in the filter cake or basemat. Specifically, any MDI additive must be stable at temperatures found in the headbox, approximately 180-205 F, and must also be stable in the ionic environment, presented in the slurry. The gypsum and various additives in the slurry produce a variety of multivalent cations. Ca+2 ions are present in a concentration of approximately 1800 ppm as a result of the solubility of the gypsum. Al+3 or K+ ions may be present depending upon the various additives used. The Al+3  ions are typically present in a concentration of 900 ppm, but may be as high as 9000 ppm. The Al+3 ions are generally derived from the accelerator alum, Al2 (SO3)4. The K+ ions are typically in a concentration of 4500 ppm, and are derived from the accelerator potassium sulfate, K2SO4. Thus, it is desirable to stabilize the MDI with an emulsifier or surfactant. The use of a stabilizing surfactant is further called for because MDI is not water soluble.
In a typical emulsion, water, MDI and a suitable surfactant are mixed under high shear so that the MDI is dispersed in the water as small droplets. The energy imparted to the system by the mixing largely controls the particle size of the resulting emulsion. In the absence of a suitable surfactant, as in the case of the premix of MDI, the dispersion of MDI particles in water eventually separates into two distinct phases. The surfactant acts to prevent this phase separation by stabilizing the individual MDI particles. On a microscopic level, the surfactant migrates to the MDI/water interface where it forms a layer with its hydrophilic portion oriented outwardly into the aqueous solution and its hydrophobic portion oriented adjacent to the MDI droplet surface. Under such a scenario, the surfactant can exhibit hydrogen bonding with the host particles, and thus remain in the basemat as the water is drawn off, rather than being siphoned out with it. In addition to hydrogen bonding, mechanical entrapment of the MDI with the gypsum and host particles also promotes a high retention of the MDI in the basement.
Surfactants are generally categorized into three classes, nonionic, cationic and anionic. Each of these surfactant classes has certain advantages and disadvantages in terms of what materials they can stabilize, how they react to high temperature, and how they react to the presence of electrolytes such as Ca+2, Al+3 or K+ ions which are present in the GWF furnish. The following are very general rules regarding the suitability of the three types of surfactants, as one skilled in the art will recognize.
Generally, nonionic surfactants are less suitable for a high temperature environment such as found in the headbox. The ethyleneoxide chains (EO) of the surfactants tend to coil upon themselves in such an environment. Under these conditions they lose their effectiveness as emulsion stabilizers. In order to increase heat stability, it is necessary to increase the hydrophobic/lipophobic balance (HLB) value of the surfactants. Nonionic surfactants are generally not affected by the presence of electrolytes, such as one might find in the GWF furnish.
Generally, anionic surfactants are not affected by high temperature and to that extent are suitable for the temperatures found in the headbox. However, certain anionic surfactants are adversely affected by multivalent cations which tend to bind with the anionic site.
Generally, cationic surfactants are not affected by either high temperature or the presence of electrolytes, and thus appear to be suitable for the GWF process. However, many cationic surfactants can contain small amounts of unreacted primary and secondary amines that can catalyze the reaction of MDI with water. This side reaction is not desired since it will lead to localized clumps of urea polymer which will not contribute to the strength of the final product. This unreacted primary and secondary amines can be scavenged by including an additive such as a Lewis acid.
Surfactants are also described in terms of how they act. An external surfactant does not react with either phase of the MDI and water solution, but simply resides at the interface between the two phases. Its presence at the interface is a dynamic process with surfactant molecules constantly leaving the interface for the bulk solution and arriving at the interface from the bulk solution.
If an external emulsifier is used, survivability of the MDI demands that the MDI emulsion utilize a high HLB nonionic emulsifier. It is known that the phase inversion temperature (PIT) varies directly with the HLB of the surfactant. Accordingly, a higher HLB emulsifier provides more heat stability to the resulting externally stabilized emulsion, and thus, the HLB value provides a good first indication of the general suitability of the surfactant. Additionally, it has been shown that cationic emulsifiers can be used in combination with high HLB nonionic surfactants to achieve suitable heat and electrolyte stability without the undesirable effects of using a cationic emulsifier alone.
An internal surfactant contains functional groups such that it actually chemically reacts with the dispersed phase. In an internally stabilized emulsion, the ethylene oxide chains are chemically attached onto the MDI backbone. By varying the molecular weight of the ethylene oxide chains (i.e., varying the number of EO groups on the chain), it is possible to adjust the “effective” HLB of the molecules.
If an internal emulsifier is used, survivability again demands that the MDI emulsion utilize a high HLB nonionic emulsifier. As discussed above, the HLB can be adjusted by varying the molecular weight of the ethylene oxide chains. Since the phase inversion temperature (PIT) varies directly with the HLB of the surfactant, a higher HLB, achieved by using more EO groups, provide s more heat stability to the resulting internally stabilized emulsion.
Thus, because of the different characteristics of each type of surfactant, one must find a single surfactant or a combination of surfactants to achieve a stabilized emulsion that provides the requisite thermal and electrolyte stability.